According to Boccato et at. (3), the halogenated end groups which may be found in the PVC are: -CHC12, -CH2C1, -CH=CHCI, -CCl=CH2. Moreover, methyl and vinyl groups may also be considered as possible chain terminations. In the infrared spectra of all investigated samples of PVC H, we did not detect any vinyl group. Our detection limit was one vinyl in 10,000 CHz, which means that on a one-vinyl-perchain basis, the number average degree of polymerization would have been higher than 5000, which is larger than the real values in our samples. We reacted the model compounds of Table I1 with LAH, under the operating conditions used for PVC. All the chlorine atoms are exchanged except the 1-vinyl ones; the double bonds are not saturated. The catalytic hydrogenation in the presence of Raney nickel and of crushed anhydrous potassium hydroxide suspended in decalin has allowed us to transform 1-chloro-1-heptene into heptane at room temperature and under atmospheric pressure. At 130 "C and under 15 bars pressure of hydrogen with the PVC H, we reached the expected increase of the per cent methyl content computed on the basis of one 1-chlorovinyl end group per chain. Although this variation ACHa % is of the same
m,
order as the uncertainty in the infrared measurement, a highly significant statistical correlation is observed between the theoretical values ACHa % = 50/DP, and the experimental ones. For a set of 23 different PVC H, we have found a coefficient of correlation of 0.71, the linear regression at the 95 % level of confidenceis: ACHI % exp = (1.04
=k
0.45) X ACH3 % theor
+ (0.002 rf 0.04)
in the range 0.01 $ A@H3 % $0.35: results are given in Table 111. We think, therefore, that it is justified to admit the existence of one single 1-chlorovinylend group per molecule of PVC. ACKNOWLEDGMENT
The authors express their thanks to Mr. J. GiudiceE who carried out the infrared part of this work and to Dr. A. J. de Vries who helped prepare the English manuscript. RECEIVED for review June 15,1970. Accepted August 5,1970.
Spectrophotometric Submicrodet ouble Extraction of the e;. R. Macchi and Be S . Cescon Osseruatorio GeoJisico Sperimentale, Viale Romolo Gessi, 4 Trieste, Italy
SEVERAL MODIFICATIONS for the spectrophotometric determination of nitrites are based on the classical Griess reaction, whereby nitrous acid is converted to a highly colored azo dye. However, that of Shinn (I) is more advantageous because of its sensitivity and easier applicability. The Shinn procedure is based on the diazotization of nitrous acid with sulfanilamide followed by the coupling with N-(1 naphthy1)ethylenediamine dihydrochloride at an approximate pH 2. According to Bendschneider and Robinson (2), who applied this method extensively to the analysis of sea water, the product of the coupling reaction is represented by Equation l
+
RE0
N H ~ S O ~ C ~ H ~ N S N CC~~ ~ H ~ N H C H Z C H2HC1 ~ N He ~.
NH2SO2CsH4N=NC~oHaNHCHzCHzNHz.2HC1 I
+ HC1
(1)
Compound I behaves as an azo dye acid-base indicator with a very sharp change from red at pH 3.7 to yellow-pink at pH 4.3 (Equation 2). The equilibrium of Equation 2 shifts to the right when Hf reacts with added base. The sequence of the compounds formed after the addition of the base is the following: HzNOzS-R-NHa(+) 111 pH 4.3-8.5
HzNOZS-R-NH~ IV pH 8.5-10.5
(-)OaS-R-NHs V pH >10.5
(1) M. B. Shinn, IND.ENG.CHEW, ANAL.ED., 13, 33 (1941). (2) K. Bendschneider and R. J. Robinson, J. Mar. Res., 11, 87 ( 1952).
Y ELLOW-PINK
Compound IV with chloroform is quantitatively extractable from the aqueous solutions and is the same yellow-pink color in both aqueous and organic phases. Since it is possible to regain the red compound 11 by extracting it with hydrochloric acid solution from the chloroform phase, a procedure which enables the improvement of the optimum concentration range for the classical determination of nitrite can be developed. This procedure is based on the extraction of compound IV with 100 ml of chloroform from a sample of 1000 ml, and also on a retrograde extraction of compound I1 with 10 ml of hydrochloric acid solution at pH 1.7. As shown in Table I, the working range of nitrite concentration has been lowered by a factor of 100, and a linear rela-
ANALYTICAL CHEMISTRY, VOL. 42,
NO. 14, DECEMBER 1970
e
1809
0.25D
with the appropriate reagent, the solution is shaken for 7 minutes with the first portion of 80 ml of chloroform. When the two phases are well separated, the chloroform layer is transferred into a dry separatory funnel of 150 ml. The operation is repeated with the second portion of 20 ml of chloroform and the total volume is extracted by shaking it for 3 minutes with 10 ml of 0.02N hydrochloric acid. The chloroform layer is then discarded and the aqueous solutiqn is analyzed spectrophotometrically at a wavelength of 5430 A.
t
0.200-
V LJ
f
0.150-
8 l4 l2
DISCUSSION
0.100
0.050
0.000 10
12
PH
Figure 1. Effect of pH on chloroform extraction Table I. Comparison between Shinn’s and Present Procedure Shinn procedure Present procedure Added Added nitrite, nitrite, pnoPe/l. Absorbance pmole/l. Absorbance 0.00 0.002 0.0000 0.047 0.25 0.052 0.0025 0.096 0.50 0.102 0 0050 0.147 0.75 0.153 0.007S 0.196 1 .oo 0 203 0.0100 0.247 I
I
Table 11. Absorbance of Replicate Standards Added nitrite concentration (pmoIe/l.) 0.0000 0.002s 0.0050 0.0075 0.0100 0.046 0.097 0.146 0.196 0.249 0,045 0.094 0.147 0.198 0.248 0.052 0.097 0.152 0.194 0.247 0.198 0.245 0.049 0,098 0.146 0.047 0.046 tionship between absorbance and concentration was verified over the range 0.000-0.010 pmole/l. of added nitrite. The mean value of the factor of enrichment was 99.2. EXPERIMENTAL
paratus. Absorbance measurements were made with an Optica CFd single beam spectrophotometer equipped with 4-cm path length cells. Reagents. All chemicals employed in reagents preparation were Carlo Erba S.p.A. analytical grade. Twice-distilled water was used throughout. pH ADJUSTMENT SOLUTION.Dissolve 10.3 grams of boric acid with 210 ml of 32 % sodium hydroxide solution and dilute with water to 1 liter. SULFANILAMIDE SOLUTION.Dissolve 5 grams of sulfanilamide in 500 ml of 1.7N hydrochloric acid. N-(1 NAPHTHYL)ETHYLENEDIAMMINE DIHYDROCHLORIDE SOLUTION. Dissolve 0.25 gram of the dihydrochloride in 250 ml of water and store in a dark bottle. The solution should be prepared daily. CHLOROFORM. Only redistilled chloroform was used. Procedure. Transfer 1000 ml of sample into a separatory funnel, add to and mix with 10 ml of sulfanilamide solution and, after 4 minutes, add 10 ml of N-(1 naphthy1)ethylenediamine dihydrochloride solution, which is allowed to react for 15 minutes. After the pH has been adjusted at a 9.5 value 1810
0
Effect of pN. Figure 1 shows the effect of pH on the extraction of cornpound IV with chloroform. The concentration of the nitrite solution was 0.01 Mmole/l., the expected absorbance was 0.200. The optimal pH interval for the quantitative extraction is in the range of 8.5-10.5, as shown in Figure 1. Neither compound 111, still present at p H IO3 are extractable with chloroform. On the other hand it is worthwhile to point out that the compound formed by employing sulfanilic acid instead of sulfanilamide as the diazoting agent shows a similar behavior and, in particular, it was proved that it is quantitatively extracted by chloroform at an approximate pH 9. Precision. Table 11 shows the results of replicate determinations of some standard samples in the range of added nitrite (0.000-0.010) pmole/l. The calculated standard deviation of absorbance measurements is =t0.0024 corresponding to a concentration value of =k0.00012 pmole/l. of nitrite. Accuracy. As it is possible to see from Table 11, at a zero concentration value of added nitrite, there is a corresponding absorbance mean value of 0.047. Since this blank, due to the reagents and to the nitrite content of twice distilled water, is not negligible when compared with the precision of our method, and since we must subtract it from the measurements in order to obtain accurate values, there is a need of evaluating the contribution of the reagents alone. This aim has been attained by treating the mixture of the reagents alone (10 ml of sulfanilamide solution 10 ml of N-(1-naphthy1)ethylene9.6 ml of pH adjustdiamine dihydrochloride solution ment solution) as described in the recommended procedure. In this case the measured absorbance had a value of 0.037. In effect, several measurements carried out with different volumes of twice-distilled water and the same amount of reagents as in our procedure, allowed us to obtain by extrapolation an absorbance value of 0.037 equal to that previously established. (During these experiments the extraction pH value remained in the correct range 9.0-10.) The accuracy is affected by two errors of the same order of magnitude, that derived from the determination of the blank of the reagents and that due to the measurement of the sample. Applicability. It seems quite reasonable to also predict the applicability of this procedure in the determination of nitrate traces, provided that they are previously reduced to nitrite form by employing either amalgamated (3) or copperized (4) cadmium reduction columns. Finally, the possibility of separating compound I1 by extraction may extend the field of application of this spectrophotometric procedure.
+
+
RECEIVED for review May 5,1970. Accepted August 12,1930. This work has been supported by the contribution of the National Research Council of Italy. (3) A. W. Morris and J. P. Riley, Anal. Chim. Acta, 29, 272 (1963). (4) E. D. Wood, F. A. J. Arrnstrong, and F. A. Richards, J . Mar. Biol. Ass. U.K . , 47, 23 (1964).
ANALYTICAL CHEMISTRY, VOL. 42, NO. 14,DECEMBER 1970
